Field of the Invention
The present invention relates to improved arrangements for providing thermal connection between a cryogenic refrigerator and cooled components, wherein the refrigerator is removable, and the thermal connection must be capable of being broken and re-made without discernable increase in thermal resistance.
Description of the Prior Art
The present invention is described in the context of a cryogenic refrigerator cooling to temperatures of about 4.2K for re-condensing helium in a cryostat used for cooling superconducting magnets for MRI systems.
FIG. 1 shows a conventional arrangement of a cryostat including a cryogen vessel 12. A cooled superconducting magnet 10 is provided within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in the vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, toward the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage 22 is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage 24 provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.
A negative electrical connection 21a is usually provided to the magnet 10 through the body of the cryostat. A positive electrical connection 21 is usually provided by a conductor passing through the vent tube 20.
Typically, the cryogenic refrigerator will be a two-stage refrigerator, providing high-power cooling to a first cryogenic temperature and lower-power cooling to a much lower cryogenic temperature, as illustrated in FIG. 1. In current cryogenic refrigerators, the first stage may provide about 44 W of cooling to 50K and about 1 W of cooling at about 4K. Typically, a first stage heat exchanger 22 is in thermal contact with the thermal radiation shield 16 as illustrated in FIG. 1.
In some conventional systems, a second stage heat exchanger is exposed to a gaseous cryogen environment in the present example, gaseous cryogen. The second stage is cooled to a temperature below the boiling point of the cryogen, which condenses onto the second stage heat exchanger. Such arrangements provide direct contact between cryogen and second stage heat exchanger, but care must be taken when removing and replacing the refrigerator, since air will tend to be drawn into the cryogen vessel, where it will freeze onto surfaces, and may cause dangerous blockages. The service operation of removing and replacing the refrigerator with the magnet at field is also a hazardous operation, as a quench could take place while the refrigerator is absent, placing a service technician at risk from exposure to liquid and gaseous cryogen.
FIG. 2 schematically illustrates a conventional arrangement in which the cryogenic refrigerator is housed within an enclosure 15, colloquially known as a “vacuum sock”, sealed from the interior of the cryogen vessel 12. In this case, the second stage heat exchanger 24 is in thermal contact with gaseous cryogen in the cryogen vessel 12 through a part 26 of a wall of the vacuum sock 15. A heat exchanger surface 28 may be provided on the cryogen vessel side of this part 26 of the wall, to enhance thermal transfer, for example having a finned and/or textured surface. Cooling in this way, by conduction through a wall of the vacuum sock, introduces thermal resistance between the second stage 24 of the refrigerator and the cryogen gas, but provides the advantage that the cryogenic refrigerator 17 may be removed and replaced without exposing the interior of the cryogen vessel 12 to air. Air may enter the vacuum sock 15, but this will solidify inside the vacuum sock when the refrigerator is in use, and does not pose a risk of dangerous blockage. The thermal connection between the first cooling stage 22 and the thermal radiation shield 16 may be provided by a tapered cooling stage 22 and a tapered interface block 30.
It is of course important to ensure effective thermal transfer between the first cooling stage 22 of the refrigerator 17 and the thermal radiation shield 16. This may be achieved, as illustrated, using a tapered first cooling stage 22 and a tapered interface block 30 which is thermally and mechanically joined to the thermal radiation shield 16. The first cooling stage 22 and the interface block 30 are each typically of copper, and the taper angle α is chosen to be narrow enough to ensure a high enough pressure between the surfaces of the first cooling stage 22 and the interface block 30 to ensure good thermal conductivity, but not so narrow an angle that the refrigerator 17 becomes difficult to remove. At an upper end of the refrigerator 17, a flange 32 is bolted 34 to the surrounding surface of the cryostat OVC 14. The dimensions of the various components are carefully calculated such that the first cooling stage 22 and the interface block 30 are driven together with an appropriate force as the refrigerator is tightened into position by bolts 34. Some flexibility in the mounting of the interface block 30 restricts the maximum force to an appropriate level, and allows for some tolerance in the respective dimensions.
Thermal connection must also be provided from the second cooling stage 24 through the wall of the vacuum sock 15. Typically, a part 26 of the wall which contacts the second stage 24 will be of a thermally conductive material such as copper, and may be profiled to provide an enhanced surface 28 for heat exchange on the side which is exposed to the interior of the cryogen vessel. For example, that surface may be finned and/or textured. In certain known arrangements, the various components are dimensioned such that the second cooling stage 24 presses in to wall part 26 with appropriate force and the tapered first cooling stage 22 meets the tapered interface block 30 with appropriate force as the flange 32 is bolted 34 on to the surrounding surface of the cryostat OVC 14. Conventionally, a compliant interface material, typically an indium washer, may be placed between mating surfaces of the wall part 26 and the second cooling stage 24 to allow effective thermal connection while allowing some tolerance in mechanical position. A difficulty with such an arrangement is that the indium washer is destroyed when the refrigerator is removed, and it is difficult to remove all traces of an old indium washer from the inside of the vacuum sock 15. Any remaining traces of an old indium washer will degrade the thermal interface provided by a replacement indium washer.
In the prior art arrangements as discussed above, efficient thermal interfaces between the refrigerator and cooled components have relied upon precise mechanical dimensions. Mechanical force applied when bolting 34 the flange 32 of the refrigerator 17 in place is shared between sealing the refrigerator to the surrounding surface of the cryostat OVC 14, and interface forces between the first cooling stage 22 and the interface block 30; in some embodiments, also interface forces between the second cooling stage 24 and the part 26 of the wall of the vacuum sock. This sharing of forces means that any mechanical tolerance in respective dimensions will change the proportions of force applied at each interface, resulting in unpredictable thermal resistances of the various interfaces. This is usually overcome at the first stage by adding additional thermal links with braids and an axial spring mechanism to allow for build tolerances at the expense of less efficient thermal transfer, caused by an increased number of thermal joints.